Mac only: Accurately named menu bar app Cinch lets you set windows up side by side, à la Windows 7's Aero Snap feature, just by dragging them to the side of your screen.
After installing Cinch, you are given the option of having it launch at login, and whether you want to see it in the menu bar. The app itself is very simple—to snap a window to half-screen size, just drag it until your cursor hits the side of the screen, and a dotted line will show you where the window will go. Releasing the mouse button will snap it into place, allowing you to have two windows side by side to work with. In addition, you can drag a window to the top of your screen to maximize it. After you're done working, drag the window away from its screen edge and its original size will be restored.
Cinch is free as nagware, costs $7 for the nag-free version, and is Mac only. Thanks, Jasmeet!
Windows: You’ve got a web page, a file folder, and a chat window open, and they’re all about the same project. WindowTabs, a free-to-try utility, can group together all those app windows with top-most, Google-Chrome-like tabs.
The free trial of WindowTabs doesn’t have a time expiration on it, but does limit you to three tabs per group. The fully-unlocked application goes for $19. That’s pretty limiting in some ways, but WindowTabs lets you pick and choose which applications it allows tabbing for, either by an inclusive or exclusive list of program executable names. So if you already use Google Chrome, or don’t think you want email windows stacked, you can add “chrome” and “outlook” to your exclude list. Alternately, you can use that three-tab limit only for applications where it would really help, like folder views and non-tabbed chat windows.